Field of the Invention
The present invention relates to an output driver circuit provided with an overcurrent protection function.
Description of the Related Art
A conventional output driver circuit with an overcurrent protection function will be described. FIG. 5 is a circuit diagram illustrating the conventional output driver circuit.
A conventional output driver circuit 50 with an overcurrent protection function includes an NMOS transistor 55, which is an output driver connected to an output terminal 57, a sense resistor 58, which is connected to a current path of the output driver and which monitors an output current, a reference voltage circuit 51, which outputs a reference voltage, a comparator 52, which compares a voltage generated at the sense resistor 58 and the reference voltage, and an NOR circuit 54, which controls the gate of the NMOS transistor 55 according to an output signal of the comparator 52 and a signal of an input terminal 53.
If the output terminal 57 and a power source are short-circuited with the NMOS transistor 55 in an ON state, then a large current flows between the drain and the source of the NMOS transistor 55. At this time, if a voltage generated at the sense resistor 58 increases to be higher than a reference voltage, then control is carried out to cause an output signal of the comparator 52 to switch to “H” level, thereby turning the NMOS transistor 55 off. This interrupts the flow of the current between the source and the drain of the NMOS transistor 55, thus protecting the IC from being damaged by an overcurrent.
Alternatively, an NMOS transistor may be connected in place of the sense resistor 58 to provide the same protection (refer to, for example, Japanese Patent Application Laid-Open No. 6-38363).
However, the conventional output driver circuit with an overcurrent protection function has been posing a problem in that the need for a comparator and a reference voltage circuit inevitably results in a large circuit scale.
Further, there has been another problem in that a sense resistor is connected to the source of an NMOS transistor, which is an output driver, thus deteriorating the drive capability of the output driver during a normal operation.